In the course of the operational life of an air separation unit, there are times when the full capacity of the plant is not utilized to make products. For example, it can be economical to shed power load to an air separation unit (“ASU”) by turning down the plant production rate during periods when electrical power is at peak value (e.g. weekday afternoons during the summer). The total flow of product from the ASU to the customer is maintained by vaporizing stored liquid product and mixing it with the product from the ASU. The liquid used during load shedding is regenerated by increasing the liquid production rates during periods when electrical power has relatively low value (e.g. weekday evenings and weekends). Load shedding is economical because the value of incremental power sold (or saved) during the peak period more than offsets the costs associated with the additional power required during the evening and the extra capital required for the capacity to remake the liquid.
In addition to load shedding, there are several other scenarios where excess ASU capacity may exist that is being underutilized. For example, there could be a facility that supplies customers that have variable demand for products and the ASU production rate would be ramped up or down in response to that demand. Or, there could be an enclave of several individual plants where an entire ASU or multiple ASUs could be turned on or off to meet customer demand.
As will be discussed, the present invention provides a method and apparatus in which, unlike the prior art, the main air compressor is not turned down during periods in which the ASU is being turned down to allow excess compressed air from the main air compressor to be used to support combustion of a fuel and thereby produce heated combustion products that are expanded in a turbine connected to a generator to generate electrical power. The electrical power can be supplied to the grid or used to power the main air compressor to either realize an economic gain from the supply of such power to the grid or to reduce the electrical power costs involved in powering the main air compressor.